Method of forming stainless steel of improved drawability

ABSTRACT

Described herein is a method of making stainless steel and the stainless steel so produced which possesses improved drawability. The steel contains not more than 6.75 percent nickel and the compositon thereof is adjusted in accordance with a prescribed equation to provide a work hardening rate of 45-85.

United States Patent [191 Hahn et al. Apr. 16, 1974 [54] METHOD OF FORMING STAINLESS STEEL 3,155,497 11/1964 Tietig 75/53 0 IMPROVED DRAWABILITY 3,174,847 3/1965 Dagan... 3,253,966 5/1966 Malagan [75] Inventors: Ronald A. Hahn, Lower Burrell; 2,534 11 19 6 Allen,,,..

Edward G. Dubrawka, New 3,366,470 1/1968 Wonus Kensington, both of Pa. 1

[73] Assignee: Allegheny Ludlum Industries; Inc.,

Pittsburgh, Pa Primary Exammer-Hyland Bizot Attorney, Agent, or Firm-Vincent G. Gioia; Robert F. Flled: NOV. 5, Dropkin [21] Appl. No.: 196,180

Related US. Application Data [63] Continuation-impart of Ser. No. 854,271, Aug. 29, [57] ABSTRACT 1969, abandoned.

52 1,5, 1 75 129, 75 23 R, 75 23 3 Described herein is a method of making stainless steel 75 123 w and the stainless steel so produced which possesses 51 Int. Cl. C22c 33/00 improved drawability- The steel contains not more 53 F of Search 75 123 N, 123 B 43, 53 than 6.75 percent nickel and the compositon thereof is adjusted in accordance with a prescribed equation 5 R ferences Cited to provide a work hardening rate of 45-851 UNlTED STATES PATENTS 2,903,386 2 Claims, No Drawings 9/1959 Waxweiler 75/128 N METHOD OF FORMING STAINLESS. STEEL OF IMPROVED DRAWABILITY This application is a continuation-in-part of now abandoned application Ser. No. 854,271, filed Aug. 29, 1969.

The present invention relates to an austenitic stainlesssteel. alloy system that has physical properties including work hardening characteristics such that it exhibits improved drawability and is suitable for temper rolling.

There are many applications in which stainless steel is used whereinisteel is subjected to considerable forming operations and quite naturally, steel of good ductility is required. A typical application of this nature is the manufacture of wheel covers for automobiles. A commonly used stainless steel alloy for this application is AlSl Type 301. As manufactured, however, this alloy contains considerable quantity of nickel, i.e., usually 7-8 percent and the present steel making operations do not consistently produce material with adequate ductility and drawability. The present invention provides a method for manufacturing stainless steel of improved drawability of stainless steel; however, arelationship is disclosed hhich has been found to be not adequately reliable for steel making operations to consistently produce compositions with the required properties. Furthermore, the equation cannot be used to balance the composition of relatively low nickel alloys so as to achieve the desired combination of strength and ductility. It has now beenidiscovered that sucha combination of properties can be achieved when the elements inthe alloy are balanced in accordance with the equation described above.

In an illustration of the practice of the invention in accordance with presently preferred embodiments, a number of samples were melted and the chemistry adjusted in accordance with the foregoing equation. The composition of these samples is given in Table l. lngots of these steels were hot-rolled to band gage and annealed. They were then cold-rolled to strip and specimens taken. Upon testing, the samples were found to have the properties described. in Table 11. As can be seen, all had work hardening rates within the -85 range of the equation. described above and contained less than 6.75 percent nickel.

cent nickel and adjusting the composition of the melt according to work hardening rate equation where W is a value in the range of 45-85:

W 2045.13 l045.82(%C) 69.29(%Mn) 96.70(%Ni) l293.63(%N The relationship defined above is a result of the discovery that the elements of the composition should be adjusted and balanced in a prescribed manner in order to achieve the required combination of strength and duetility necessary for many drawing applications. The term work hardening rate" refers to increase in stress with increasing strain. It has been found that when the work hardening rate is below 45, there is insufficient martensite transformation to obtain uniform elongation with the result that the steel will unduly decrease in cross-section (i.e., neckdown) signaling the onset of fracture. At work hardening rates of over about 85, there is a tendency to build up high stress levels which lead to premature fractures; that is, the material possesses less ductility and lower elongation. In the preferred embodiment,- the chemistry of the steel is adjusted to achieve a work hardening rate in the range of -80.

In an article which appeared in the August, 1964 issue of Metals Progress (pages 115 118) the author described the effect of work hardening on deep Heat No. c Mn P 5 Si Cr Ni Mo Cu N,

MT20..... .147 1.30 .016 .010 56 16.60 6.20 .21 .17 5 0 MT22..... .130 1.33 .016 .010 54 16.70 6.56 .21 .18 .050 MT-24..... .122 1.33 .012 .010 55 16.40 6.40 .20 .17 .049 MS65..... .118 1.39 .016 .010 .50 16.68 6.58, .21 .19 .033

dr awability with consistently good properties, and with TABLE I] lower nickel than has heretofore been used. The re- Hard- .2 Elong. duced nickel content reduces the cost of the steel and Percent UTS H tN 30T Y the balanced composition of the invention provides 35 ea 0 Sksl ks] (percent) w consistently good drawability MT-20 69.5 40.0 it 134.0 56.6 77.1 I d .th th th d MT22 68.0 39.3 123.6 63.0 63.7- n accor ance n e mven 1011, ere 1S provi e MT 24 675 400 123.0 623 628 a method of making stainless steel which involves pre- MS-65 68.0 37.3 131.0 61.1 t 74.4 paring a melt of steel having not more than 6.75 perln manufacturing steels in accordance with the invention, it is recognized that although it may be necessary to make specific additions of certain elements present in the alloy, some of the elements may be present in residual quantities within the prescribed ranges necessary to conform to the work hardening rate equation. Also, other elements are normally present as residuals in steel making operations and these include, for example, copper, molybdenum and. sometimes cobalt. Steels of the type of composition described above have been successfully produced and manufactured into wheel covers and have excellent drawability.

Steels for which the practice of the invention is particularly well suited are those consisting essentially of,

in weight percent, up to 0.15% carbon, up to 3% manganese preferably 0.45 to 1.8%, 16 to 18% chromium, 5 to 6.75% nickel, generally 6 to 6.75% nickel, up to 0.1% nitrogen, up to 1% silicon and up to 0.5% each molybdenum, copper and cobalt and the balance iron and steel making residuals.

It is apparent from the foregoing that various changes and modifications may be made without departing from the invention, accordingly, the scope thereof should be limited only by the appended claims wherein what is claimed is:

We claim:

1. A method of making stainless steel of improved drawability and suitable for temper rolling which com- 3 4 prises preparing a steel melt consisting essentially of up W 2045.13 1045.82 (%C) 69.29 (%Mn) 64.49 to 0.15% carbon, up to 3% manganese, 16 to 18% chro- (%Cr) mium, to 6.75% nickel, up to 0.1% nitrogen, up to 1% silicon, up to 0.5% each of molybdenum, copper and 96.70 (%Ni) 1293.63 (%N cobalt, and the balance iron and steel making residuals 5 and adjusting the composition thereof to provide a bal- 2. A method according to claim 1 wherein W is a anced steel composition in accordance with the following work hardening rate equation where W is a value in the range of 45 85:

value in the range of 50 80. 

2. A method according to claim 1 wherein W is a value in the range of 50 -
 80. 